My 2016 Sierra SLE has similar issues. However, I found a temp solution that I’ll continue to use until GM produces a solution (not holding my breath). I purchased an aftermarket mod that plugs in to the OBD port. It does not modify the tune but it disables the AFM feature. The engine never switches from 8 cylinder mode. I have the 5.3L and my gas mileage has not changed in a measurable amount. I drive 100 miles per day round trip in varied city/freeway traffic and I’m seeing 16 – 18 mpg average over 400 miles.
I can confirm that it did not flag on my truck when I took it to the service center last week for the airbag recall service. Before I left home, I unplugged the device, started the truck and drove to the center. This is the procedure detailed in the user info for this mod.
I left it unplugged until the work was done and I was leaving. They never mentioned it and I was in and out in about an hour.
The point is, that without the constant changes to cylinder configuration, the transmission shifts better. I’ve also read that after a few months of of this, you can stop using the mod and let it go back to normal mode with AFM. By this time the transmission has “learned” how to shift. There might be some minor degradation because of AFM but eventually, the transmission computer can compensate for the change in profile and smooth that out too. I cannot personally attest to this however.
